Update README with architecture and deployment details

[bitflicker64]

Description

Added Architecture section to README.md providing:

• Module breakdown table (core engine, datasets, utilities, visualization, dev tools)
• Deployment modes (Tomcat/Jetty/Docker)
• Architecture highlights (40+ dataset types, format-agnostic I/O, aggregation, caching)

Helps new contributors understand ERDDAP's structure at a glance.

Fixes # N/A

Type of change

• This change requires a documentation update

Checklist before requesting a review

• I have performed a self-review of my code
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes